Alloy films preparation

Thus nickel and nickel-copper alloy films evaporated in vacuo onto the inner walls of the reaction vessel have been chosen for further investigations. The films were deposited onto the inner wall of a glass tube kept at 450°C their thickness amounted to approximately 2000 A. After annealing at the same temperature in vacuo they were transferred into the side-arm of the Smith-Linnett apparatus in order for the recombination coefficients to be determined. The bulk homogeneity of alloy films prepared in this way was confirmed by X-ray diffraction (65, 65a, 66). 

In discussing the principles involved in alloy film formation, reference had to be made to alloy systems which are uncommon or unused in studies of adsorption and catalysis. This section is specifically concerned with the characterization of alloy films prepared for such purposes. However, the various aspects of alloy film structure mentioned in Section II have to be kept in mind when discussing results of catalytic experiments using evaporated alloy films. 

The use of hydrogen annealing to ensure homogenization and surface cleanliness is an attractive procedure in alloy film preparation but for the question of surface enrichment. The possibility of surface enrichment in... 

When Pd-Rh alloy films (prepared by simultaneous deposition at 400°C)... 

Fig. 30. Work function of Pt-Au alloy films prepared by simultaneous deposition at — 196°C measured at room temperature ( ) and after sintering at 300°C (O) (41).

There is now available a substantial amount of information on the principles and techniques involved in preparing evaporated alloy films suitable for adsorption or catalytic work, although some preparative methods, e.g., vapor quenching, used in other research fields have not yet been adopted. Alloy films have been characterized with respect to bulk properties, e.g., uniformity of composition, phase separation, crystallite orientation, and surface areas have been measured. Direct quantitative measurements of surface composition have not been made on alloy films prepared for catalytic studies, but techniques, e.g., Auger electron spectroscopy, are available. 

High-Performance Soft Magnetic Alloy Films Prepared by Electrodeposition... 

Fig. 50. Schematic representation of alloy film preparation by means of sputtering.

Takahashi et al. [220] first reported the formation of Bi-Te alloy films with varying chemical composition by means of cathodic electrodeposition from aqueous nitric acid solutions (pH 1.0-0.7) containing Bi(N03)3 and Te02. The electrodeposition took place on Ti sheets at room temperature under diffusion-limited conditions for both components. In a subsequent work [221], it was noted that the use of the Bi-EDTA complex in the electrolyte would improve the results, since Bi " is easily converted into the hydrolysis product, Bi(OH)3, a hydrous polymer, thus impairing the reproducibility of electrodeposition. The as-produced films were found to consist of mixtures of Te and several Bi-Te alloy compounds, such as Bi2Tc3, Bi2+xTe3 x, Bi Tee, and BiTe. Preparation of both n- and p-type Bi2Te3 was reported in this and related works [222]. 

It is reasonable to ask two questions in relation to studies using evaporated alloy films, viz, why work with alloys and why prepare alloy catalysts in this particular form ... 

The pure metals are readily available to try out as catalysts, whereas the alloys are not, if a moderate surface area is required. Methods for preparing alloys as high-area powders, etc., raise questions about the unwanted introduction of promoters, e.g., chloride ions. Here, again, evaporated alloy films recommend themselves for the exploration of a whole new territory of alloy systems as catalysts in a variety of reactions. 

There are a variety of options available for the preparation of evaporated alloy films and, indeed, suggested methods of preparation can be traced back over many years (9). Nevertheless, it is possible to distinguish some general methods for the preparation of binary alloys as follows ... 

A convenient starting point is to consult the phase diagrams in Hansen s Constitution of Binary Alloys (10) and supplementary volumes (11,12), but information may also be needed on miscibility at the temperatures normally employed for film preparation and catalytic reaction. Therefore, some consideration must be given to the thermodynamic properties of the... 

The above work suggests that various factors might determine the extent of alloying and that the result would be specific to the experimental arrangements adopted, e.g., the amount of radiant heat will vary. Therefore, it seems unwise to rely on alloy formation during the deposition of the second layer and, in fact, Cu-Ni films prepared for surface studies by... 

It is particularly helpful that we can take the Cu-Ni system as an example of the use of successive deposition for preparing alloy films where a miscibility gap exists, and one component can diffuse readily, because this alloy system is also historically important in discussing catalysis by metals. The rate of migration of the copper atoms is much higher than that of the nickel atoms (there is a pronounced Kirkendall effect) and, with polycrystalline specimens, surface diffusion of copper over the nickel crystallites requires a lower activation energy than diffusion into the bulk of the crystallites. Hence, the following model was proposed for the location of the phases in Cu-Ni films (S3), prepared by annealing successively deposited layers at 200°C in vacuum, which was consistent with the experimental data on the work function. 

Finally, with respect to successive evaporation, Pd-Rh films used for CO oxidation (34) are an example of preparing alloy films where a miscibility gap exists and interdiffusion rates are slow. These Pd-Rh films were prepared by depositing layers of palladium and rhodium at 0°C, followed by annealing in 50 Torr hydrogen at 400°C for 21 hr. The apparent surface compositions, evaluated from the CO oxidation rate as described in Section IV, and information on film structure obtained by X-ray diffraction (XRD) are recorded in Table II. 

Values of P/MUi are tabulated by Holland (44), but wre provide in Table III further values ( calculated from data in Smithells (45) ] which enable us to discuss some recent observations on the preparation of alloy films, most of which were used in catalytic experiments. 

The diffusion coefficients of palladium in a Pd-Ag alloy and silver in a range of Pd-Ag alloys are known, and the diffusion of palladium and silver atoms in a 20% Pd-Ag alloy was calculated (30) for t = 3600 sec representing the film preparation time. At temperatures of 100°, 200°, 300°, and 400°C, silver atoms would diffuse in this time distances of 3 X 10-4, 0.15, 9, and 150 A, respectively whereas at the corresponding temperatures, palladium atoms would diffuse 26, 460, 3000, and 11,000 A. Palladium atoms can thus penetrate the alloy lattice at moderate temperatures, whereas silver atoms have a probability of diffusing distances equivalent to a few unit cells only when the substrate temperature is greater than 300°-400°C. 

Table IV shows X-ray data (55) on the homogeneity of Pd-Ag films prepared by simultaneous evaporation from separate sources, either in conventional vacuum or in UHV, with the substrate maintained at 0°C. The second group of films was prepared using a stainless steel system incorporating a large (100 1/sec) getter-ion pump, sorption trap, etc., but deposited inside a glass vessel. By the tests of homogeneity adopted, alloy films evaporated in conventional vacuum were not satisfactory, i.e., the lattice constants were generally outside the limits of the experimental error, 0.004 A, and the X-ray line profiles were not always symmetrical. In contrast, alloy films evaporated in UHV were satisfactorily homogeneous. Further, electron micrographs showed that these latter films were reasonably unsintered and thus, this method provides clean Pd-Ag alloy films with the required characteristics for surface studies.

In this context, it is interesting to note that it has been claimed (56) that single-crystal Fe-Ni alloy films can be prepared by deposition on heated rock salt substrates in vacua of 10-3-10-4 Torr. Other workers (57) have found that the use of UHV permits single-crystal films of Fe-Ni to be formed (at deposition rates of 14 A/min) without the annealing necessary after deposition at 1(U5 Torr. Single-crystal Au-Pd films have also been prepared (58) and after quenching from 500°C gave an electron dif-... 

Vapor quenching provides a method of bridging the miscibility gap which exists in many alloy systems, and makes a range of novel alloys available for study. Such films, of course, would not be ideal for catalytic studies. They could not be used at high temperatures, and indeed the heat of reaction might be sufficient to induce a transformation to a more stable structure. In addition, characterization by X-ray diffraction would be difficult, even for the crystalline films, because of line broadening by the small crystallites. Nevertheless, alloy films which are metastable above room temperature can be prepared, and their high surface area would... 

Figure 7 (a, b, d, and e) shows transmission electron micrographs from Pd-Ag films of comparable weight, prepared and annealed at 400°C, and used once to catalyze the oxidation of ethylene at 240°C (40). The structure of this series of alloy films varied consistently with composition. Silver-rich films (e.g., Fig. 7a, 13% Pd) showed extensive coalescence of the crystallites, while at the other end of the composition range (e.g., Fig. 7e,...

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